Do you use all the points for tracking? In the AF menu you can select "only cross type AF point". This will reduce the number of points for tracking but will give you more precise focus and more keepers. Also as it has been said if you fill your frame with your subject then it easier for the camera to find a spot to focus on. For sports you should also enable the back AF button and use the shutter button just to take the picture while using the back AF button to track.